1. Field of the Invention
The present invention relates to a system for monitoring and controlling the rate of fluid flow from an injection well used for in-situ remediation of contaminated groundwater.
2. Discussion of Background
Contaminated soil and groundwater can be treated in a direct manner by excavating contaminated subsurface materials such as soils, sediments, fractured rock, and the like, and by pumping contaminated water to the surface of the earth for treatment. These methods of dealing with contaminated groundwater are relatively expensive.
An indirect method relies on well systems for extraction of contaminated groundwater or for injection of various treatment substances for in-situ stabilization. An injection well for in-situ treatment of contaminated groundwater in a well system, and in particular a horizontal well system, is described in commonly assigned U.S. Pat. No. 4,832,122 issued to Corey, et al. The system disclosed therein by Corey is illustrated in FIG. 1.
As seen in FIG. 1, the subsurface structure under the earth's surface 10 includes an upper, unsaturated or vadose zone, indicated generally at A, and a lower, saturated zone, indicated generally at B. Zones A and B meet at water table 12. Plume 14, which contains contaminants having a preference for the gaseous phase, lies below surface 10. Injection well system 16, situated below plume 14 in saturated zone B, includes vertical shaft 18, horizontal injection well 20, and pump 22. Extraction well system 24, situated above plume 14 in vadose zone A, includes vertical shaft 26, horizontal extraction well 28, and pump 30. Pump 30 is connected to treatment device 40.
Horizontal wells 20 and 28 have spaced multiple apertures 42, such as slots or perforations. Apertures 42 are large and numerous enough to allow fluid to flow freely therethrough, but narrow enough to keep soil particles from blocking the flow. Alternatively, wells 20 and 28 may be surrounded by mesh sleeves (not shown) to prevent blocking of apertures 42 by soil particles but allow the free flow of fluid out of well 20 and into well 28.
As noted above, factors such as the subsurface geology of the area, fluid flow rates, size and shape of the plume, and drilling economics dictate the dimensions, configuration and orientation of the two well systems, as will be apparent to one skilled in the art. Horizontal wells up to several hundred feet long have been used to treat contaminated groundwater.
Fluid 44 is pumped into injection well system 16 by pump 22. Fluid 44 flows through vertical shaft 18 to injection well 20, exits through apertures 42 into saturated zone B, and percolates into plume 14. The volatile contaminants in plume 14 are carried by fluid 44 to extraction well system 24, entering extraction well 28 through apertures 42. Fluid 44, carrying the volatilized contaminants from plume 14, is drawn by pump 30 to treatment device 40 where the contaminants are separated from fluid 44. Purified fluid 44 may be recycled to injection well system 16 or dispersed into the atmosphere.
The system works well in relatively homogeneous soil. The treatment fluid is injected uniformly into the plume and percolates upwards at approximately the same rate along the entire length of the injection well. However, flow is not uniform if the subsurface conditions are not uniform. In areas of varying permeability, rocky soil, subsurface fissures, mixed soil types, etc., the fluid tends to find preferential pathways through the soil. More fluid passes through some regions of the plume and these regions are decontaminated faster than others. While the overall rate of fluid flow into the injection well can be controlled through pumping volume and aperture size and number, the differential rate of flow into the soil along the length of the well is in major part a function of subsurface characteristics beyond the control of the well operator. Treatment must be continued until the entire plume is decontaminated. Therefore, treatment must continue for a longer time, using more materials, and at a greater cost than in areas of uniform subsurface conditions.
Methods are available for controlling overall fluid flow in injection wells. U.S. Pat. Nos. 4,691,778 and 3,993,130 use two concentric cylinders with multiple ports which can be selectively aligned to adjust the size of the openings, thereby controlling the injection profile. An apparatus for consolidating loose sand around an injection well uses pressure-responsive check valves to regulate the injection of predeterminable amounts of consolidation fluid into a plurality of spaced-apart perforations in the well casing (U.S. Pat. No. 3,362,477).
For efficient treatment of a contaminated plume, it would be desirable to monitor the amount of fluid flowing out of the injection well and differentially adjust the flow so that the amounts injected along the length of the well are appropriate for the subsurface conditions, including the soil conditions and the levels of contaminants, to optimize the overall efficiency of the process. Furthermore, it is desirable to change the flow rates over time if the conditions warrant, such as if the water table fluctuates or if the plume spreads. There is, however, no available method for differentially adjusting the flow of fluid through multiple openings in a horizontal injection well system.